Fast Application Programming Interface, better known as FAPI, is a standard developed by the LTE Femto Forum in which exchange of information between the two layers is performed via certain message structures. 4G systems of today, such as those that use the 3GPP Long Term Evolution (LTE) standard, can make use of this FAPI standard in order to exchange configurations between different layers in the radio software stack, as well as reporting indications and errors upward in the stack. FAPI, like other APIs, is a set of routines, rules or protocols that are used to build or connect software components. The FAPI standard provides a clear picture of how a LTE-based layered system is interconnected within itself and how information exchange is carried out as a whole.